The Genetics of P. infestans

Date: Monday, June 17, 1996
From: p.gregory@cgnet.com

Potato blight, caused by the fungus Phytophthora infestans, is the most important constraint to potato production worldwide. Historically, the disease is best known for the terrible human suffering it caused in Ireland in the 1840's. Nearly a million people died of starvation and another 3 million emigrated as a result of crop failure caused by late blight. Late blight can completely defoliate a crop in a matter of a few days and eventually rots the tubers.

For nearly one hundred years, control of late blight has involved use of chemical fungicides that are expensive and can be injurious to health and the environment. These compounds are known to affect the soil flora and fauna, and have been linked to breast cancer in women. In the 1990's, the use of fungicides increased dramatically because the late blight fungus is overcoming its susceptibility to some of the most widely-used chemical controls. This means that in the future, farmers will likely spray more than ever before, though with more limited results.

The escape from Mexico of the A2 mating type of P. infestans raises the specter of even higher virulence of the pathogen and disturbing changes in the epidemiology of the fungus. What the world's farmers have been battling for the past 150 years is the A1 mating type of the pathogen. Conventional wisdom holds that both the A1 and A2 types originated in the Toluca Valley of Mexico. But for reasons that we do not fully understand, the A2 remained behind while the A1 spread around the world.

The situation changed dramatically in the 1970s when the A2 was unintentionally exported to Europe on contaminated Mexican potatoes. Most scientists agree that it was then re-exported on infected seed potatoes. Were you to take a map of the countries where the A2 is present and overlay it with a map of countries that purchase seed potatoes from Europe, you would find you have an almost perfect match.

The problem comes to a head when A1 and A2 are present in the same environment and recombine sexually. The result is an explosion of genetic variation of the fungus and the enhanced probability that even more virulent forms of the disease will be produced. The new forms of the disease could be resistant to some of the most powerful fungicides and to existing sources of genetic resistance within the crop. Moreover, due to the formation of oospores, which are the product of sexual reproduction between the A1 and A2 types, late blight could become an earlier blight and begin infecting the plant immediately after it emerges. Oospores can survive between cropping seasons and become active soon after planting. With just the A1 present, infestation usually takes place quite late in the season when the disease is easier to manage. Earlier infection means that farmers are battling the problem soon after emergence.

The general implications of this development are likely to result in dramatic increases in the use of fungicide sprays and the development of even more powerful chemicals, both of which carry important consequences for the environment and health. In addition, farmers face substantial annual increases in the costs of production. Nowhere, however, are the challenges likely to be greater than in the developing world where farmers often lack the cash required to buy chemical inputs and where potatoes are helping to meet the future food challenges that the Consultative Group on International Agricultural Research (CGIAR) is trying to address.

According to a recently completed book published by the FAO crop statistics unit and the International Potato Center (CIP), annual potato production in lesser developed countries (LDC) has grown from 30 to 85 million tons since the early 1960's. Developing country production is currently growing at an annual rate of 3.6%, a figure that is expected to hold steady for some time. By the year 2000 developing countries will be producing more than a third of the world's potato crop. These figures are extremely conservative and are likely to increase as developing countries diversify their agriculture. We also expect that the unexploited yield potential of potatoes will be called upon to play a far greater role in feeding LDC populations as we begin to bump against the yield ceiling of the most popular cereals. Currently, developing country potato yields average about 13 tons/hectare, but most of these countries could easily produce upwards of 40 tons/hectare. Most potato experts agree that yield potential of the crop can reach 120 tons/hectare.